1. Field of the invention
This invention relates generally to networks that receive and transmit groups of logic signals from remote locations and, more particularly, to apparatus for insuring the accuracy of the information transmitted between the local and the remote locations.
2. Description of the Related Art
In the automatic control of processes, process control networks are provided to collect status information for use by at least one central location and to distribute control signals to the devices coupled to the network. Referring to FIG. 1, a portion of such a process control network is shown. The process control network includes a system bus 5 which has at least one central control unit 6 coupled thereto and at least one bus interface module 9 also coupled thereto. The central control unit 6 receives signals from the system bus 5, signals that typically communicate the status of user devices coupled to the process control network and applies signals to the network that provides the control signals determining the operation of the user devices. The bus interface module 9 applies status signals to the system bus and receives signals from the system bus 5 which are relevant to a subset of network user devices 10 that communicate with the central control unit 6 through bus interface module 9. User devices 10, (such as motors, lamps and heaters), responsive to digital signals, receive digital control signals through device interface unit 16 and digital output module 12 from the bus interface module 9. User devices 10, generating digital status signals in response to contact closures, transmit the status signals to the bus interface module 9 by means of device interface unit 17 and digital input module 13. User devices 10, (such as control valves), responding to analog control signals, receive signals from the bus interface module 9 by means of analog output module 14 and device interface unit 18. User devices 10, (such as flowmeters, level detectors, pressure meters etc.), of the type generating analog status information transmit the status signals to the bus interface module 9 through device interface unit 19 and analog input module 15. The device interface units 16, 17, 18 and 19 convert the signals received from and transmitted to the user devices into a format usable by the digital output and input modules and the analog output and input modules, respectively. The digital input and output modules 13 and 12 and the analog input and output modules 15 and 14 couple the signals of the user devices, as transformed by the device interface unit, with the bus interface module 9 by means of local bus 11.
In the communication between the bus interface module 9 and the digital and analog input and output devices, the messages typically have an upper bound on the time that a single message can be active (in the preferred embodiment, the upper limit is 10 milliseconds [ms], however, this interval is a matter of design choice). A malfunction in the network can result in uncontrolled communication of information, generally referred to as jabber, in which the message can extend beyond the single message boundary. Because the jabber is the result of a malfunction, such a condition must be detected and the network repaired or inactivated. The detection of a jabber event is typically performed by an antijabber timing unit which measures the duration of the message. However, the antijabber circuit can itself malfunction and not detect a jabbering incident. Furthermore, in process control networks of the related art, a jabber incident can be the result of a single component malfunction.
A need has therefore been felt for apparatus and an associated method that can verify the operation of the antijabber timing circuit without compromising the normal communication operation, and which can assist in isolating a malfunction that could result in a jabber event, the jabber event resulting in interruption of communication on the network beyond the immediate consequence of the malfunction.